"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Kuo: AirPods With Wireless Charging Case Expected in Early 2019, All-New Model in Early 2020
We attribute AirPods' dramatic growth (14–16mn, 26–28mn, 50–55mn, 70–80mn and 100–110mn units in 2017, 2018, 2019, 2020, and 2021, respectively) to 1) the upgraded model with wireless charging support to benefit shipments to launch in 1Q19, 2) the all-new design model to boost replacement demand to launch in 1Q20, 3) an innovative user experience, and 4) high integration with iOS and Mac products. In summary, AirPods is Apple's most popular accessory ever, and it currently has the best growth momentum among Apple products.Kuo notes that fewer than 5 percent of iPhone users currently own AirPods, and with over 1 billion iPhone users worldwide, there are "great potential growth opportunities" for the accessory.
Nearly a year ago, Kuo predicted Apple would launch upgraded AirPods in the second half of 2018, but Apple's difficulties in bringing its AirPower charging mat to market have undoubtedly impacted its AirPods plans to some degree. Just ahead of Apple's October media event, Kuo noted he was uncertain about the timing for an AirPods update, indicating it could come late in the fourth quarter or early in the first quarter of 2019.